1.Technical Field of the Invention
The present invention relates to mobile telecommunication systems, and more particularly, to a method and apparatus for channel estimation within high data rate mobile communication systems.
2. Description of Related Art
The growing area of personal communication systems is providing individuals with personal terminals capable of supporting various services such as multimedia. These services require the use of increased bit rates due to the large amount of data required to be transferred. The use of increased bit rates generate problems in conventional single carrier systems due to intersymbol interference (ISI) and deep frequency selective fading problems.
One solution to these problems utilizes orthogonal frequency division multiplexing (OFDM) within the radio mobile environment to minimize the above-mentioned problems. Within OFDM, a signal is transmitted on multi orthogonal carriers having a bandwidth less than the coherence bandwidth in order to combat frequency selective fading of the transmitted signal. The ISI is mitigated by the use of guard intervals. OFDM systems are presently adopted in Europe for digital audio broadcasting and has been proposed for use in digital TV broadcasting systems. It is used also in the asymmetric digital subscriber lines (ADSL) to transmit high rate data.
Existing channel estimation methods are based on adaptive signal processing wherein the channels are assumed to vary slowly. The estimated channel parameters at a particular time depend on the received data and channel parameters at a previous time. In the case of fast varying channels, such as in high data rate mobile systems, these methods must be modified to reduce the estimation time. Existing modified channel estimators are complex in implementation and use time pilots or frequency pilots to estimate the impulse response or frequency response times. For deep fading channels, linear estimations will have poor performance. However, nonlinear estimators will provide better performance. Thus, a need exists for an improved method for channel estimation for orthogonal frequency division multiplexing systems in frequency selective fading mobile channel.
In OFDM, channel equalization is moved from the time domain to the frequency domain to enable the estimation of the channel frequency response. In time-invariant channels, commonly used equalization methods use zero forcing or least mean square (LMS) algorithms. The equalizer used in time-invariant channels is not necessarily suitable for the time variant channels such as the frequency selective fading channels which are characteristic of mobile communication channels. Several methods have been proposed for channel estimation and equalization for use with OFDM.
One method utilizes a periodic broadcasting of a block of train pulses as a time pilot. The time interval between each pulse is greater than the channel delay spread. However, the implementation is complex and requires additional circuitry for channel estimation and noise carrier suppression. Another method uses conventional LMS algorithms and additional tuning coefficients to speed up the manipulation of the equalizer. A modified method is used to compensate for the amplitude and phase errors common to all carriers in certain regions by using separate common carrier amplitude and phase estimators for some frequency plans. The equalizer structure for this method is also quite complex.